In recent years, in accordance with a rapid spread of information relevant apparatuses and communication device such as a personal computer, a video camera and a portable telephone, the development of a battery to be utilized as a power source thereof has been active. The development of a high-output and high-capacity battery for an electric automobile or a hybrid automobile has been advanced also in the automobile industry. A lithium ion battery has been noticed from the view point that energy density is high among various kinds of batteries.
Conventionally, improving performance of a lithium battery while focusing on an interface of an electrode active material has been attempted in the field of such a lithium battery. For example, in Patent Literatures 1 to 3 and Non Patent Literatures 1 and 2, it is disclosed that a coating layer is formed on the surface of a cathode active material of a lithium battery.
Specifically, in Patent Literature 1, a technique for coating a lithium ion conductive oxide such as Li4/3Ti5/3O4, LiTi2 (PO4)3 or LiNbO3 on the surface of a cathode active material such as LiCoO2 or LiMn2O4 by using a sol-gel method is disclosed. In Patent Literature 2, a technique for coating Li3PO4 and Li4SiO4 on the surface of a cathode active material composed of LiCoO2 by using a PLD method is disclosed. In Patent Literature 3, a technique for coating ZrO2, Al2O3, TiO2, or the like on the surface of a cathode active material composed of LiNiCoMnO2 by using a barrel sputtering method is disclosed. In Non Patent Literature 1, a technique for coating Li3PO4 on the surface of a cathode active material composed of LiNi0.5Mn1.5O4 by using an electrostatic atomizing method is disclosed.
Incidentally, in Non Patent Literature 2, it is disclosed that both Li4SiO4 and Li3PO4 are so high in lithium concentration that it is difficult to coat them on the surface of a cathode active material by using a sol-gel method or a quenching method.